1. Field of the Invention
The present invention relates to data communications techniques which are used in voiceband telephone networks.
Voiceband telephone networks are often used as a transmission medium for data communication using modems. Most telephone networks in use today employ digital transmission. However, since the signals generated by conventional telephones and modems are analog rather than digital, conversion between the two formats is necessary, and is usually performed at telephone company central offices. Analog signals from the customer premises are transmitted to the central offices over wire pairs called `loops`. At the central office, the analog loop signals are sampled and converted to a digital data stream--a sequence of digital words--for transmission through the network. Conversely, digital data streams arriving at the central office from the network are converted back into analog form for analog transmission over the loop to the appropriate customer.
Telephone systems throughout the world have adapted various standards for encoding the analog signals as digital data streams. Illustratively, the Public Switched Telephone Network (PSTN) in use throughout the U.S. utilizes an encoding scheme in which the analog line signal is bandlimited to approximately 3 kHz, then sampled every 125 microseconds (8000 samples per second) with each sample being mapped to one of 255 possible quantization levels according to a non-linear mapping rule called .mu.-law. The quantization level chosen for each sample is the one closest to the sample voltage, and this level is then represented as an 8-bit digital word. Thus, the analog line signal is represented as a 64 kilobit/second data stream composed of 8000 8-bit words per second. This conversion process, and the reverse process of converting the digital stream back to analog form, is performed at the central office by a device known as a coder/decoder, or CODEC.
The analog-to-digital conversion process performed by the CODEC inherently introduces distortion into the signal. This distortion, called quantization distortion, is due to the mapping of the analog voltage sample into the nearest quantization level. The .mu.-law rule was designed to permit voice signals, which have a dynamic range of considerably more than 8 bits, to be encoded into 8-bit samples while maintaining a level of quantization distortion which is not perceptually objectionable.
In addition to voice, telephone system customers may also utilize the network to transmit and receive digital data using modems. A modem performs the task of converting the customer's digital data sequence into a bandlimited analog signal which can be transmitted through the telephone network as though it were a voice signal. A modem at the destination customer premises receives the analog modem signal and converts it back into the transmitted digital sequence. A key point here is that known conventional modems, in effect, treat the telephone network as though it were a pure analog channel, i.e. they do not explicitly take into account the effects of analog-to-digital and digital-to-analog conversion within the telephone network, and so the analog signals generated by such modems suffer the same quantization distortion effects as do ordinary voice signals. This distortion, called quantization noise, though not large, nevertheless represents a significant impairment as modem transmission speeds are increased. Primarily due to the limiting effects of quantization noise, present art conventional modems operate at a maximum channel data rate of around 20 kilobits/second (kbps).
A central concept of the present inventive modem is that the effects of quantization distortion can be avoided by using pulse amplitude modulation (PAM) in which the amplitude levels are chosen to be the quantization levels used by the telephone network. The user's data is encoded into this quantization-level channel symbol alphabet, and thus explicitly transported across the network in digital form. We refer to a modem utilizing this principle as a `quantization level sampling` (QLS) modem.
2. Information Disclosure Statement
One of the impediments to the realization of a QLS modem has been the lack of an appropriate framework, both theoretical and practical, for attacking the significant equalization problems involved, since the methods used in conventional modems are not directly applicable to the QLS equalization problem. However, we have realized that aspects of the theory of equalization in multi-user communication systems, such as described in B. R. Petersen and D. D. Falconer, "Minimum Mean-Square Equalization in Cyclostationary and Stationary Interference--Analysis and Subscriber Line Calculations", IEEE JSAC, Vol. SAC-9, No. 6, pp. 931-940, can be applied in a heretofore unrealized manner to the QLS equalization problem. Thus, while multi-user data communications systems do show the underlying theory of one aspect of the present inventive modems, there has not heretofore been any use or application of that technology to single user modems as in the present invention.